1. The Field of the Invention
The present invention is in the field of shaped, flexible dental trays used to deliver a dental treatment composition to a person's teeth. More particularly, the invention relates to flexible dental trays that can be injection molded from a thermoplastic polymer mixed with a plasticizer.
2. The Relevant Technology
Virtually all people desire white or whiter teeth. To achieve this goal, people have veneers placed over their teeth or have their teeth chemically bleached. A common bleaching method involves the use of a dental tray that is custom-fitted to a person's teeth and that is therefore comfortable to wear. One type of customized tray is made from a stone cast of a person's teeth. Another is customized directly using a person's teeth as a template (e.g., “boil-and-bite” trays). Non-customized trays that approximate the shapes and sizes of a variety of users' dental arches have also been used. A dental bleaching composition is placed into the tray and the tray placed over the person's teeth for a desired period of time.
Another bleaching method involves painting a bleaching composition directly onto a person's teeth. A perceived advantage of paint-on bleaching is that it eliminates the need for a dental tray. The main disadvantage of a paint-on bleaching composition is that it remains directly exposed to the person's saliva and disruptive forces found in a person's mouth. As a result, a significant portion of the bleaching composition does not remain on the teeth where bleaching is desired. Some or all of the composition can dissolve away into the person's saliva and/or be transferred to adjacent oral tissues, potentially irritating soft oral tissues.
Another tooth bleaching method involves placing a flexible bleaching strip over a user's tooth surfaces. Conventional bleaching strips comprise a flexible plastic strip coated with a dental bleaching gel of moderate viscosity and relatively low stickiness on the side of the strip facing the user's teeth. To install the bleaching strip, a portion of the bleaching strip is placed over the front surfaces of the user's teeth, and the remainder is folded around the occlusal edges of the teeth and against a portion of the lingual surfaces. Like paint-on bleaching compositions, this procedure does not require the use of dental trays. Unlike paint-on bleaching compositions, bleaching strips include a plastic barrier that, at least in theory, keeps the dental bleaching gel from diffusing into the user's mouth.
In reality, because of the generally poor adhesion of bleaching strips to the user's teeth, coupled with their generally flimsy nature, it is often difficult for the user to maintain the bleaching strip in its proper position for the recommended time. Even if a user successfully maintains a conventional bleaching strip in its proper position during the recommended bleaching period, the bleaching gel often diffuses into the person's saliva, potentially causing a poor taste in the user's mouth and possibly discomfort to soft oral and throat tissues. The tendency of the bleaching gel to diffuse into the user's mouth can be accelerated through even minimal shifts of the bleaching strip over the user's teeth, with each shift potentially causing bleaching gel that remains adhered to the user's teeth, but not covered by the plastic strip, to be exposed to saliva in the user's mouth. In some cases, the bleaching strip can become so dislodged or mangled that it must be removed by the user and replaced with a fresh bleaching strip to complete the recommended bleaching time. This multiplies the cost and hassle of using conventional bleaching strips.
Ultimately, the main impediment to successful bleaching is the failure of users to complete the prescribed bleaching regimen. If the bleaching apparatus is difficult to install over a person's teeth, requires numerous repetitions to achieve observable results, or is uncomfortable to wear, the user may simply give up and prematurely abort the prescribed bleaching regimen. Thus, even if dental bleaching is possible using a particular bleaching apparatus or method, it is less likely to occur if the inadequacies of the bleaching apparatus or method cause a user to become discouraged before desired results are attained.
The thickness of a dental tray is often an important characteristic in determining the resulting comfort of the tray. Thinner trays provide increased flexibility and conformability. The manufacture of thin-walled trays by vacuum forming a polymer sheet is more expensive and time consuming than injection molding. Whereas injection molding is a viable process for manufacturing thicker-walled trays and mouth guards, it has heretofore not been possible to injection mold trays thinner than about 0.015 inch. As a result, trays having a thickness less than 0.015 inch must still be vacuum formed from an initially flat sheet.
It would be an improvement in the art to provide improved thermoplastic  compositions that permit dental trays having thicknesses less than about 0.015 to be injection molded while having sufficient flexural strength to resist collapse after being demolded. Such an improvement would provide a low cost, high quality dental tray with increased flexibility and conformability, which would be expected to improve or encourage compliance to a treatment regimen by the user.